Usually, PIN (P-type Intrinsic N-type) photo diode is used as a photo sensor, because the thickness of its depletion region (i.e. Intrinsic region) could be modified to get the best quantum efficiency and frequency response.
Refer to FIGS. 1 and 2. FIG. 1 shows the structure of a conventional PIN photo diode, and FIG. 2 shows the energy band diagram of a conventional PIN photo diode. In FIG. 1, an anti-reflection glass layer 10 is used for accepting the incoming light. Most of the incoming light may penetrate it without being reflected. A transparent conducting layer 11 is formed on the anti-reflection glass layer 10. Generally, it's an indium tim oxide layer. Then a P type amorphous silicon, an intrinsic type amorphous silicon layer 13 and an N type layer 12 silicon layer 14 are formed sequentially on the anti-reflection glass layer 10. After that, an aluminum metal contact 15 is formed on the N type amorphous silicon layer 14. When a reverse bias is applied on the device, the intrinsic type amorphous silicon layer 13 is totally depleted. Referring to FIG. 2, electron-hole pairs are generated, and separated by strong electric field when the incident photons are absorbed. These two kinds of carriers cross the depletion region to form photo current. Thus, photo energy is transferred into electrical energy.
However, the deficiency of conventional PIN photo diode is that the generated photo current is generally very small, therefore another amplifier circuit is necessary to enlarge the detected photo current.